1. Field of the Invention
The present invention relates to the treatment of petroleum coke containing therein valuable metals, particularly vanadium and nickel. Petroleum coke is a material of low volatility produced during the refining of crude oil and generally comprises carbonaceous material including elemental carbon, as well as relatively heavy hydrocarbon products including straight- and branched-chain saturated and unsaturated hydrocarbons, cyclic and polycyclic saturated and unsaturated hydrocarbons, whether unsubstituted or substituted with acyl, cyano, sulfur, or halogen constituents, and organometallic compounds. As used herein, the term "carbonaceous material" will mean elemental carbon, hydrocarbons and mixtures thereof. Petroleum coke that can be treated in this invention includes not only conventional petroleum coke produced by refining operations such as cracking, reforming, and the like of crude oil and tars, but also residues remaining after conventional petroleum coke is subjected to secondary treatment to recover some of the hydrocarbon content of the coke as gaseous or liquid products.
During the refining of crude oil, compounds of vanadium and nickel in the crude oil can concentrate in the petroleum coke, which thereby represents a potentially valuable source of these metals. Vanadium and nickel have many commercial uses, particularly as alloying elements of steel. Various processes have been proposed in which these metal values are concentrated or recovered from a variety of feed materials, but such processes often suffer from one or more drawbacks including unacceptably low yields, excessive consumption of energy, difficulty of separating the various constituent metals from the metal-bearing product fraction, and generation of undesirable compounds in waste effluents.
2. Description of the Prior Art
Several patents describe processes for treating vanadium-bearing ore with alkaline reagents to form alkaline vanadate solutions, but the disclosed processes do not suggest the treatment of metal-bearing material which is predominantly carbonaceous, nor do they suggest the treatment of such material in a controlled process that generates its own energy requirements and recovers selectively therefrom both vanadium and nickel in separate product streams.
For instance, U.S. Pat. No. 4,296,073 describes a process in which an ore of uranium, vanadium, and/or molybdenum is mixed with an aqueous liquor containing a major amount of sodium bicarbonate and only a minor amount of sodium carbonate, and the mixture is digested under an oxygen pressure of 0.1 to 20 bars (i.e., about 0.1 to about 20 kg/cm.sup.2) at a temperature of 160.degree. C. to 300.degree. C. The disclosure is particularly concerned with ores having a relatively high proportion of carbonate compounds but only well under 1% of organic carbon; indeed, the patentee states that such organic compounds can advantageously be removed from the metal-bearing material by a calcining step prior to the disclosed leaching treatment. In addition, the feed material contains only 0.01% or less of vanadium (as V.sub.2 O.sub.5).
U.S. Pat. No. 3,712,942 discloses the extraction of vanadium from raw material containing V.sub.2 O.sub.3 or another insoluble form of vanadium by leaching the raw material with a caustic solution containing 250 to 400 gpl of Na.sub.2 O or K.sub.2 O at a temperature of at least 250.degree. C. under an oxygen or compressed air pressure of at least about 120 atmospheres (about 124 kg/cm.sup.2). The vanadium in the raw material is oxidized to a soluble vanadate species and dissolves in the leaching solution. The disclosure does not relate to the recovery of any other metal, such as nickel, from the vanadium-bearing feed material, nor does it suggest the treatment of feed material containing any quantity of carbonaceous matter.
U.S. Pat. No. 3,429,693 discloses a process for extracting vanadium from ores and concentrates thereof by leaching the feed material with an aqueous solution up to 50% of which is an equimolar mixture of sodium carbonate and sodium bicarbonate, at a temperature of 60.degree. C. to 250.degree. C. for 30 to 480 minutes. The patentee discloses that the ore should be preliminarily roasted to convert the vanadium to its fully oxidized state (V.sub.2 O.sub.5). There is no disclosure of the presence of carbonaceous matter in the ore, but one would expect the amount to be at most quite small.
U.S. Pat. No. 3,132,920 discloses converting a hydrated oxide of tetravalent vanadium contaminated with iron, aluminum and manganese to pentavalent vanadate by leaching the hydrated oxide in an alkaline liquor while passing an oxidizing gas through the liquor. There is no recognition of the effect of iron on the extraction yield, and since there is no disclosure that carbonaceous matter is present in the starting material, there is no recognition of how to treat such matter. In addition, the patentee states that alkali carbonate is not as effective as alkali hydroxides.
U.S. Pat. No. 4,243,639 discloses the gasification of petroleum coke in the presence of an alkali metal salt such as sodium carbonate at a temperature above about 1000.degree. F. (i.e., above about 538.degree. C.) to produce a hydrocarbon gas stream and a residue containing water-leachable alkali metal vanadate. This process operates with essentially no liquid water phase, whereas as will be seen below, the process of the present invention requires the presence of a liquid water phase. The disclosed process also achieves little, if any, oxidation of the carbon in the petroleum coke starting material, and leaves a hydrocarbon residue from which the vanadium values still must be extracted.